Global positioning systems, such as the US NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Department of Defense is a satellite-based radio navigation system that transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with similar precision. GPS uses eighteen to twenty-four satellites that may be evenly dispersed in three inclined twelve-hour circular orbits chosen to ensure continuous twenty-four hour coverage worldwide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations. Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHz and an L2 signal at a frequency of 1227.6 MHz. The L1 and L2 signals are biphase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference (i.e., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations. The result is a precise determination of the location of the receiver in three-dimensional space. Velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The measured differences are used to solve another set of equations to determine the velocity based upon the Doppler phase shift of the received signal.
The utility of the GPS for guidance applications is well recognized. For military applications, GPS allows self-guided weapons to find targets with heretofore unknown degrees of accuracy. Unfortunately, GPS guidance systems use 10 watt signals from satellites in an eleven thousand nautical mile orbit. Consequently, such GPS systems are notoriously prone to interference, particularly man-made interference and RF jamming. Such compromises to GPS systems can adversely affect the navigation and precision of GPS-aided weapons. The susceptibility of GPS receivers to interference therefore necessitates an effective system for alleviating such problems.
GPS anti-interference capability has been developed in the past, but has exhibited performance limitations. Such limitations have to do with classic receiver architectures involving RF mixers and synthesizers. Specifically, for example, such classic architectures are expensive and require precision components and tuning.
Accordingly, there is a strong need in the art for a low cost system and method for overcoming the effects of interference in a GPS system.